Formalize design review facilitator role? and document how to facilitate design review

Design review offers an opportunity for an open editing process brought by any contributor and is a knowledge sharing opportunity. Intentionally is it not run as a consensus-seeking decision forum. That said, we do need clearer established guidance as to how it should operate, and it seems this could be accomplished through two changes:
1) Write a guide/document on how to facilitate the design review; and
2) Designate somebody responsible for facilitating the meeting according to the published guidance

Some of the reasons that we should do (1) are to:
- Make explicit the way that design review operates for newcomers and frequent attendees alike
- Have consistency in how design reviews operate in the event that facilitators change

In a recent TSC meeting, some further specific suggestions of what a guide ought to contain were expressed:
- [Design review process](https://github.com/ampproject/amphtml/blob/master/contributing/design-reviews.md) is documented. There should be a summary of specific guidelines the facilitator should take note to see are followed, e.g. that designs are submitted by Monday prior.
- There are potential norms or protocols that encourage fairness like addressing design review topics in the chronological order they are submitted.
- The guide might speak to special circumstances like allowing for the handling of more urgent matters that need to be reviewed.
- There could be fixed protocols such as: (1) A brief summary of how design review is run that is stated at the beginning of every design review, to be more welcoming to newcomers; (2) an explicit call out for people who are new to the meeting to introduce themselves.